This invention relates generally to ultrasound diagnostic systems that use ultrasonic transducers to provide diagnostic information concerning the interior of the body through ultrasound imaging, and more particularly, to ultrasonic membrane transducers used in such systems.
Ultrasound diagnostic imaging systems are in widespread use for performing ultrasonic imaging and measurements. For example, cardiologists, radiologists, and obstetricians use ultrasonic diagnostic imaging systems to examine the heart, various abdominal organs, or a developing fetus, respectively. In general, imaging information is obtained by these systems by placing an ultrasonic transducer array against the skin of a patient, and actuating one or more elements located within the array to transmit ultrasonic energy through the skin and into the body of the patient. In response to the transmission of ultrasonic energy into the body, ultrasonic echoes emanate from the interior structure of the body. The returning acoustic echoes are converted into electrical signals by the one or more elements in the array, which are transferred to the diagnostic system by a cable coupling the diagnostic system to the transducer array.
Recent advances in software and digital technologies have permitted the development of ultrasound imaging systems of increased flexibility and rapid data processing rates. Consequently, the number of ultrasonic transducer elements within the diagnostic probe has also steadily increased, allowing the development of relatively wide aperture diagnostic probes that yield high lateral resolution.
Acoustic transducers commonly used in ultrasonic diagnostic probes are generally comprised of an array of individual piezoelectric elements formed from a crystalline piezoelectric material by performing a number of meticulous manufacturing steps. For example, prior art acoustic transducer arrays have been formed by bonding a single block of a piezoelectric material to an acoustic backing member having a relatively low acoustic impedance and high acoustic attenuation. The material is then laterally subdivided by cutting or dicing the material to form the fine rectangular elements of the array. Electrical contact pads are deposited on the individual elements using various metallization processes to permit electrical conductors to be coupled to the individual elements of the array. The electrical conductors are generally coupled to the contact pads by a variety of electrical joining methods, including soldering, spot-welding, or by adhesively bonding the conductor to the contact pad.
Although the foregoing manufacturing steps are generally adequate to form acoustic transducer arrays having up to a few hundred elements, larger arrays of elements having smaller element sizes are not easily formed using the foregoing technique. Consequently, various methods used in the fabrication of silicon microelectronic devices have been adapted to produce ultrasonic transducer elements on semiconductor substrates, since these techniques permit the repetitive fabrication of small structures with intricate details. As a result, transducer elements that are much smaller than those attainable using the foregoing method may easily be fabricated in large numbers.
FIG. 1 is a partial cross sectional view of a micro-formed membrane ultrasonic transducer array 1 according to the prior art. The array 1 includes a plurality of micro-formed membrane elements 2 positioned on an upper surface 12 of a silicon substrate 3. The elements 2 generally include an upper surface 4 that is spaced apart from a lower surface 7 that abuts the substrate 3. Each element is coupled to a time-varying voltage source through a plurality of electrical interconnections formed in the substrate 3. For clarity of illustration, the voltage source and the electrical interconnections to each of the elements are not shown. When the time-varying excitation voltage is applied to the elements, a vibrational deflection in the upper surface 4 is developed that stems from the electromechanical properties of the element. Accordingly, acoustic waves 5 are generated that radiate outwardly from the upper surface 4 in response to the applied time-varying voltage. The electromechanical properties of the elements similarly allow the elements to be responsive to deflections resulting from acoustic waves 6 that impinge on the upper surface 4.
One disadvantage in the foregoing prior art array 1 is that a portion of the ultrasonic energy developed by the elements 2 may be projected backwardly into the underlying substrate 3 rather that being radiated outwardly in the acoustic wave 5. Consequently, a partial loss of radiated energy from the elements 2 results. Further, when ultrasonic energy is coupled into the underlying substrate 3, the ultrasonic energy propagates into the substrate 3 as an acoustic wave 8 that may be internally reflected at a lower surface 11 of the substrate 3 to form a reflected wave 9 that is directed towards the upper surface 12 of the substrate 3. A plurality of reflected waves 9 may then propagate within the substrate 3 between the upper surface 12 and the lower surface 11. A portion of the energy present in each reflected wave 9 may also leave the substrate 3 through the surface 11, to form a plurality of leakage waves 10. The propagation of acoustic waves 9 in the substrate 3 further permits ultrasonic energy to be cross-coupled between the elements 2 since the waves 9 may be received by other elements 2 in the array 1, thus generating undesirable xe2x80x9ccross-talkxe2x80x9d signals between the elements 2, as well as other undesirable interference effects. Still further, the internal reflection of waves in the substrate 3 may adversely affect the acceptance angle, or directivity of the array 1.
Various prior art devices have included elements that impede the propagation of waves in the substrate. For example, one prior art device employs a plurality of trenches between the elements 2 that extend downwardly into the substrate 3 to interrupt wave propagation within the substrate 3. A similar prior art device employs a downwardly projecting trench, which is filled with an acoustic attenuation material to at least partially absorb the energy in the reflected waves 9. Although these prior art devices may reduce the undesired lateral wave propagation in the substrate, they also generally limit the advantages inherent in micro-forming the elements 2 by reducing the number of design parameters that may be independently varied. Furthermore, the additional manufacturing steps significantly increase the manufacturing cost of micro-formed transducer arrays.
A further disadvantage associated with the prior art array 1 shown in FIG. 1 is that the array 1 is commonly exposed to a variety of substances, including water, sterilants and coupling gels. Since the elements 2 are micro-formed on the substrate 3, various small recesses may exist on the array 1 where these substances may become lodged. For example, a plurality of recesses 13 may be formed between the elements 2 that may allow the progressive accumulation of contaminants, due to the general inability to adequately clean the small recesses 13 of the array 1. Additionally, surfaces exposed to the substances may be susceptible to corrosion or erosion stemming from this exposure.
Still another disadvantage present in the prior art array 1 is that the array 1 lacks a hard cover surface to protect the array 1. Consequently, the array 1 is generally susceptible to damage resulting from physical impacts, since an impact sustained by the array 1 may damage individual elements 2, as well as the underlying substrate 3 that contains electrical interconnections or other devices.
Accordingly, there is a need in the art for micro-formed ultrasonic arrays that are capable of producing significant reductions in acoustic wave propagation in the underlying substrate. In addition, there is a need in the art for micro-formed ultrasonic arrays that resist contamination and damage from a variety of substances, and are easily cleaned. Still further, there is a need in the art for micro-formed ultrasonic arrays that can resist damage from commonly-encountered physical impacts.
The invention is directed towards improved structures for use with micro-formed membrane ultrasonic transducer arrays and methods for fabricating the improved structures. In one aspect of the invention, the micro-formed membrane transducer array includes a planar member having a layer of a piezoelectric material and a plurality of spaced apart electrodes disposed on a surface of the planar member and coupled to the layer of piezoelectric material for applying an electric field to the layer, and an acoustic backing member fixedly joined to the plurality of electrodes. In another aspect, the micro-formed membrane transducer array includes a planar member having a layer of a piezoelectric material and an adjoining layer of a semiconductor material, the layer of semiconductor material having a plurality of monolithically formed active circuits formed in the semiconductor layer and coupled to the layer of piezoelectric material, a plurality of spaced apart electrodes disposed on a surface of the planar member and coupled to the active circuits for applying an electric field to the piezoelectric layer, and an acoustic backing member fixedly joined to the plurality of electrodes. In still another aspect, the micro-formed membrane transducer array includes a planar member having a layer of a piezoelectric material, a plurality of spaced apart electrodes disposed on a surface of the planar member for applying an electric field to the piezoelectric layer, and an acoustic backing member having an adjoining layer of a semiconductor material, the layer of semiconductor material having a plurality of monolithically formed active circuits, the active circuits being coupled to the electrodes and fixedly joined to the electrodes at selected sites.